That's because the AfD
is far right, of the worst possible kind. Their members keep using Nazi and even SS propaganda, excusing Nazi and SS actions during WW2, ... (SS was Hitler's private security service, the people actually executing the holocaust), they openly discuss adopting a version of Nazi ancestry laws, ... you know, I'll make a list
* Prominent AfD figures promote a "völkisch" concept of German identity based on ancestry rather than citizenship, and propose to make laws based on this introducing what is generally referred to as "apartheid": 2 different legal statuses for German (and European) citizens based on ancestry
* The AfD "promotes remigration" (mass deportation) with some members supporting proposals that woud mass-deport German citizens. AfD figures attended the 2023 Potsdam meeting where far-right activists discussed controversial "remigration" proposals (meaning they discussed it with actual people loudly proclaiming they are Nazis)
Or how about: they keep getting caught defending Nazis, you know the actual historical party and what they did
* AfD politician Björn Höcke was convicted twice for using "Alles für Deutschland," a banned Nazi SA slogan
* Alexander Gauland described the Nazi era as mere "bird shit" in German history
* Björn Höcke called Berlin's Holocaust memorial a "monument of shame" and demanded a reversal of Germany's remembrance culture
* Maximilian Krah claimed that not all members of the Nazi Waffen-SS were criminals.
* More general, AfD politicians have maintained documented links to far-right nationalist movements (meaning actual groups calling themselves publicly Nazis)
People that have this as a job keep accusing, sometimes with VERY worrying proof, parts and people of the party of being Nazis
* Several AfD state branches have been classified as confirmed right-wing extremist by German constitutional-protection authorities
* "Junge Alternative", was classified as confirmed right-wing extremist by German intelligence
* Police arrested eight suspected members of the "Saxon Separatists," including an AfD local politician, over allegations that the group planned to use violence to establish a territory based on Nazi ideology
* Police raided AfD district politician Philip Steinbeck's property and discovered firearms, including reportedly fully automatic WWII weapons, and substantial quantities of explosives
THAT, and other reasons, is why AfD is described as extreme right. It's not that other parties don't get into controversies, but nothing remotely close to this, plus it keeps repeating.